Still available only to Insiders (i.e. on Redstone) and still pitched at being locked down by the time the 'Anniversary Update' gets rolled out, Facebook's new Messenger UWP is advancing nicely, with an update available now that sees faster start-up, proper groups support, proper handling of a virtual Windows navigation bar, and many fixes.
As before, testing of this Facebook Messenger Beta seems to be somewhat region-specific. If not already installed on your phone, the trick as usual is to to set your region to 'France'. I'm sure there are other countries where it also works, but this was the one pointed out to me! Restart the phone to pick up the new region and then go to this link in the Store on your phone. You'll be able to download and install the new UWP in the usual way.
Don't forget to restore your phone's region to its original country after installation - don't worry you'll still get application updates without having to keep flip-flopping.
Some before/after screens of the new Facebook Messenger (Beta):
Note the new 'Groups' tab along the bottom of the interface - this should have been there all along, of course, friend groups is an important part of the Messenger experience.
Before this update, you could dismiss the navigation bar and the interface would stay the same, with just a black hole - now (right) the interface expands appropriately.